Current controlled electronic circuits are regulated by sensing a load current, comparing the sensed load current to a predetermined reference and adjusting the load current accordingly to match the predetermined reference. Typically, the current sensing is performed by providing a sense resistor within the load current path and sensing the voltage drop across the sense resistor generated by the load current flowing therethrough. This voltage drop is typically small to limit the power loss due to current sensing. For current sensing at the switch pin of a half bridge pulse width modulated (PWM) driver, the switch pin voltage changes from the high side rail to ground and back every switch cycle and so the voltage measured at the two terminals of the sense resistor has a very large common mode voltage due to the switch pin voltage changes and a relatively small differential voltage relating to the sense resistor voltage drop. In order to analyze the sense current, the differential voltage across the sense resistor must be extracted from the common mode voltage present equally at both ends of the sense resistor. Ideally, this would be accomplished with a differential amplifier exhibiting a very high common mode rejection ratio (CMRR).
Unfortunately, differential amplifiers designed to amplify a very small differential voltage with a very high common mode voltage present are difficult to implement. Therefore, the high common mode voltage, i.e. in excess of 50V, should be reduced using a voltage divider, while the small differential signal of a few hundred millivolts requires differential gain. Resistor mismatch in the differential voltage dividers and differential gain scaling resistors results in common mode signal amplification which must be minimized. This could be accomplished by providing extremely accurate resistors, however this significantly increases the cost of the amplifier.